1. Field of the Invention
The present invention relates to the field of data-over-voice (DOV) transmission of data in telephone switching systems. More specifically, it relates to the field of protocol synchronization of data packets between distant devices in a distributed switching system, where the distant devices may take on a peer-peer or a master-slave relation.
2. General Background
Telephone switching systems, which connect individual telephones, carry voice information (data) from station to station. They are generally constructed to carry their data in analog form, using the frequency range from 0-3 KHz. Single-building systems often use twisted-pair or similar quality wiring to carry the data, usually with available frequency ranges of up to about 1.8 MHz. This clearly leaves a great deal of unused spectrum space available for other communication activity, such as carrying digital data. When data is carried over a communication link ("commlink") in the higher frequencies above those allocated to voice, the switching system is said to utilize data-over-voice ("DOV").
Telecommunication switching systems can be configured either point-to-point (two end devices directly communicating) or via a central switch. Because each of these forms is desirable under its own optimal set of constraints, it is perferred to have a communication method which operates correctly in either configuration.
An important aspect of communication which many systems encounter is that of synchronizing the frames in which messages are sent. Frames must be synchronized so that messages sent by one device do not collide with those sent by the other device.
A second important aspect of communication is allocation of transmission time on the commlink. Some systems allocate transmission time on the commlink by a method called time compression modulation (TCM) or "pingpong". The ping-pong method is to alternately send packages of bits ("message packets") alternately in opposite directions. The message packet must be collected in memory at one speed and transmitted at a greater speed, generally about twice the collection speed. Upon reception, the packet is decompressed to its normal data rate.
A third important aspect of communication is acknowledgement of messages to assure they have been delivered. In a distributed communication system, it is important to determine if any messages are lost in transmission, so they can be retransmitted if necessary.
3. The Prior Art
A typical method of solving the frame-synchronization problem is to designte one of the two devices as controlling this aspect of communication. This presents a problem when point-to-point communication is desired, because there must be prior agreement as to which device is controlling. Also, multiple types of devices (e.g., "master" and "slave" versions) may be required, at greater expense.
One problem which ping-pong systems are prone to is a general lack of ability to dynamically allocate transmission capacity between communication devices. Often two communication devices have, at differing times, differing requirements for the amount of data which they need to transmit. Failure to allocate transmission time to account for these differing requirements can result in waste, due to a system which operates below capacity.
One method of the prior art is to require that one device be in control of the commlink and that the commlink protocol for allocating transmission capacity must be master-slave in nature. In a multi-device communication system, this is a severe constraint, as no commlink can support two controlling devices. This requires that a single device be placed in control of the entire network. The present invention is able to allocate the commlink between two "peer" devices (neither has absolute control over the commlink) and is able to switch dynamically between peer-peer and master-slave operation in its allocation activity.
One method of solving the lost-message problem is to tag messages with identifiers and to require acknowledgement signals that the messages are received. When messages are lost, however, prior art methods generally require that one of the two devices take control and resynchronize the commlink. Again, in point-to-point communication, this presents a problem.